Do not allow empty SQL as input for SQLDatabase.execSQL()

A issue is reported here: https://github.com/cloudant/sync-android/issues/7 was caused empty "String" is used as input for "SQLdatabase.execSQL()". To fix the issue:

1  In, "SQLDatabaseFactory.executeStatements()", empty SQL String will be skipped. 
2 In both "AndroidSQLite" (on Android) and "SQLiteWrapper", non-empty String is a pre-condition now.